1. Field of Invention
The present invention relates to methods for cleaning fine coal by means of froth flotation. More specifically, the present invention relates to chemical conditioning steps for enhancing the floatability of fine coal while at the same time depressing pyrite and other contaminants by flotation.
2. Prior Art
The future effective utilization of coal as an energy source will depend largely on the development of effective techniques for separation of ash and sulfur in an economical process. Otherwise, restrictions against SO.sub.2 emissions will result in high cost for energy production, and will dictate in favor of other fuels rather than coal. The United States Department of Energy projects that by 1990 at least 54% of the electricity produced in the United States will be generated from coal. It is generally acknowledged, however, that this can only occur if processes are developed which enable the effective cleaning and production of compliance coal while at the same time providing for rejection of ash and sulfur.
With increased emphasis on maximum coal production, industry is now looking for total utilization of the coal, including coal fines and small particulate coal which has previously been discarded Further, the production of a high quality, clean coal product may require grinding to fine particle size to achieve complete liberation. Coal flotation is one process which has been applied to cleaning coal fines for commercial use. Whereas in 1950, only a few flotation plants existed in the United States, 66 plants had developed flotation production by 1970. Currently, virtually all new preparation plants incorporate flotation into their plant design. In terms of production, coal flotation plant capacity in the United States has grown from 64,000 tons per day in 1975 to 145,000 tons per day in 1985. Despite the increased commercial interest, however, the separation of ash and sulfur from coals still remains a major challenge to developing cost effectiveness in the froth flotation method.
Froth flotation is a physicochemical separation process that depends on the attachment of hydrophobic particles to air bubbles Other hydrophilic particles are wetted by the aqueous phase and will not attach to air bubbles. Thus, the separation of coal particles from gangue minerals occurs as air bubbles are dispersed through a suspension of coal particles (typically minus 28 mesh). The bubble/particle aggregates float to the surface and are collected as clean coal concentrate.
An unfortunate physical property of sulfur, and in particular pyrite, is its tendency to respond in the flotation process in the same manner as does the coal. In other words, those techniques which lead to enhanced flotability of coal also lead to enhanced flotability of pyrite Conversely, those processes applied to depress the flotation of pyrite frequently lead to coal depression.
For example, the flotation process usually involves the use of suitable reagents, such as neutral molecular oils, to enhance the hydrophobic character of coal particles, while the gangue mineral particles remain hydrophilic. These neutral oils such as kerosene or fuel oil are called promoters and are used to enhance the attachment of air bubbles at the coal surface. This is done by forming a thin coating of promoter over the air bubble and/or particle to be floated. In addition, frothing agents such as methylisobutyl carbinol, terpinol, creosols, polyglycols, and some specially blended reagents are used. The choice of these reagents and level of addition depends on the coal to be floated and the desired level of selectivity with respect to ash and sulfur. Because pyrite from coal has some tendency to float, use of these agents tends to cause their flotation along with coal, destroying the clean coal product quality.
Where fine coal is subjected to the flotation process, greater amounts of promoter and frother agent are adsorbed due to high surface area. In fact, the liberated fine mineral matter itself attaches to the hydrophobic coal particles, resulting in a slime coating with an attendant pseudo-depression phenomenon. As a result of these complications, the production of super clean or even compliance coal by conventional froth flotation has been a most difficult task. Although some success has been achieved utilizing sodium hypochlorite for removal of sulphatic and organic sulfur, such oxidation practice has been generally unsuccessful in the removal of pyritic sulfur. These problems are most significant for coals such as medium volatile bituminous, high volatile bituminous and sub-bituminous coals.
It has been well known for many years that natural occurring coal tends to be hydrophobic. In fact, higher grade coals are extremely hydrophobic and need very little treatment to improve their amenability to flotation. With respect to the medium and lower grade coals, the natural hydrophobic character is decreased, particularly for the high volatile bituminous and sub-bituminous categories. Furthermore, the greater the ash content in the coal, the less hydrophobic is the material.
It is also generally known from the literature that surface oxidation of coal in most cases further decreases its hydrophobic character and leads to a poorer flotation response (S. C. Sun, Trans AIME, vol. 6, No. 4, p. 396, 1954; S. K. Chakrabartty and N Berkowitz, Fuel, vol. 53, p. 240, 1974; F. F. Aplan, Flotation, M. C. Fuerstenau editor, AIME, New York, p. 1235, 1976; R. R. Yarzab, Z. Abdel-Baset, and P. H. Given, Geochimica et Cosmochimica Acta, vol. 43, p. 281, 1979; D W. Fuerstenau, J. M. Rosenbaum, and J. S. Laskowski, Collids and Surfaces, vol. 8, p. 153, 1983; D. W. Fuerstenau, G C. C. Yang, and J. S. Laskowski, Coal preparation, vol 2, p. 1, 1986). This generally acknowledged fact is further evidenced by U.S. Pat. No. 4,452,714 by McCarthy In fact, the McCarthy patent teaches the use of reducing agents to eliminate oxidized surfaces of the carbon for improvement of flotation. Similarly, U.S. Pat. No. 4,537,599 by Greenwald, Sr. teaches that "the oxidized surfaces of the coal particles are so altered that separation of tailings from the coal particles cannot be carried out by conventional means such as froth-flotation," column 2, lines 43-47. The oxidizing agent used in the Greenwald discussion was ozone. The Greenwald patent further discloses the teachings of U.S. Pat. No. 4,328,002 which discusses a process for treating coal to remove sulfur and ash which involves the steps of preconditioning coal particles in the presence of an oxidizing agent The Greenwald patent indicates that such oxidants as H.sub.2 O.sub.2, HNO.sub.3, HCLO.sub.4, HF, O.sub.2, air and mild NH.sub.3 or CO.sub.2 are substantially ineffective to provide useful results in flotation processes. The reference further points out the problem that froth flotation cannot be used to separate ultra fine impurities that are freed by the action of the oxidants with respect to the carbon particles. It concludes that the known process of using an oxidant in coal flotation does not provide for separation of these impurities from coal particles less than 105 microns in size.
Accordingly, what is needed is an effective process for enabling the separation of pyritic sulfur and other contaminants from fine coal as part of an economical froth flotation process.